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Page 1: Java Virtual Machine (JVM ) - instructional media + magic | Virtual Machine 1 Java Virtual Machine A Java Virtual Machine (JVM) enables a set of computer software programs and data

Java Virtual Machine 1

Java Virtual MachineA Java Virtual Machine (JVM) enables a set of computer software programs and data structures to use a virtualmachine model for the execution of other computer programs and scripts. The model used by a JVM accepts a formof computer intermediate language commonly referred to as Java bytecode. This language conceptually representsthe instruction set of a stack-oriented, capability architecture. Sun Microsystems states there are over 4.5 billionJVM-enabled devices.[1]

OverviewA JVM can also implement programming languages other than Java. For example, Ada source code can be compiledto Java bytecode, which may then be executed by a JVM. JVMs can also be released by other companies besidesOracle (the developer of Java) — JVMs using the "Java" trademark may be developed by other companies as long asthey adhere to the JVM specification published by Oracle and to related contractual obligations.Java was conceived with the concept of WORA: "write once, run anywhere". This is done using the Java VirtualMachine. The JVM is the environment in which java programs execute. It is software that is implemented onnon-virtual hardware and on standard operating systems.JVM is a crucial component of the Java platform, and because JVMs are available for many hardware and softwareplatforms, Java can be both middleware and a platform in its own right, hence the trademark write once, runanywhere. The use of the same bytecode for all platforms allows Java to be described as "compile once, runanywhere", as opposed to "write once, compile anywhere", which describes cross-platform compiled languages. AJVM also enables such features as automated exception handling, which provides "root-cause" debugginginformation for every software error (exception), independent of the source code.A JVM is distributed along with a set of standard class libraries that implement the Java application programminginterface (API). Appropriate APIs bundled together form the Java Runtime Environment (JRE).

Execution environmentPrograms intended to run on a JVM must be compiled into a standardized portable binary format, which typicallycomes in the form of .class files. A program may consist of many classes in different files. For easier distribution oflarge programs, multiple class files may be packaged together in a .jar file (short for Java archive).The Java application launcher, java, offers a standard way of executing Java code. Compare javaw.[2]

The JVM runtime executes .class or .jar files, emulating the JVM instruction set by interpreting it, or using ajust-in-time compiler (JIT) such as Oracle's HotSpot. JIT compiling, not interpreting, is used in most JVMs today toachieve greater speed. There are also ahead-of-time compilers that enable developers to precompile class files intonative code for particular platforms.Like most virtual machines, the Java Virtual Machine has a stack-based architecture akin to amicrocontroller/microprocessor. However, the JVM also has low-level support for Java-like classes and methods,which amounts to a highly idiosyncratic memory model and capability-based architecture.

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Java Virtual Machine 2

JVM languages

Versions of non-JVM languages

Language On JVM

Erlang Erjang

JavaScript Rhino

PHP Resin

Python Jython

REXX NetRexx[3]

Ruby JRuby

Tcl Jacl

Languages designed expressly for JVM

MIDletPascal

Clojure

Groovy

Scala

Although the JVM was primarily aimed at running compiled Java programs, many other languages can now run ontop of it.[4] The JVM has currently no built-in support for dynamically typed languages: the existing JVM instructionset is statically typed,[5] although the JVM can be used to implement interpreters for dynamic languages. The JVMhas a limited support for dynamically modifying existing classes and methods; this currently only works in adebugging environment, where new classes and methods can be added dynamically. Built-in support for dynamiclanguages is currently planned for Java 7.[6]

Bytecode verifierA basic philosophy of Java is that it is inherently "safe" from the standpoint that no user program can "crash" thehost machine or otherwise interfere inappropriately with other operations on the host machine, and that it is possibleto protect certain methods and data structures belonging to "trusted" code from access or corruption by "untrusted"code executing within the same JVM. Furthermore, common programmer errors that often lead to data corruption orunpredictable behavior such as accessing off the end of an array or using an uninitialized pointer are not allowed tooccur. Several features of Java combine to provide this safety, including the class model, the garbage-collected heap,and the verifier.The JVM verifies all bytecode before it is executed. This verification consists primarily of three types of checks:• Branches are always to valid locations• Data is always initialized and references are always type-safe• Access to "private" or "package private" data and methods is rigidly controlled.The first two of these checks take place primarily during the "verification" step that occurs when a class is loadedand made eligible for use. The third is primarily performed dynamically, when data items or methods of a class arefirst accessed by another class.The verifier permits only some bytecode sequences in valid programs, e.g. a jump (branch) instruction can only target an instruction within the same method. Furthermore, the verifier ensures that any given instruction operates on

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Java Virtual Machine 3

a fixed stack location,[7] allowing the JIT compiler to transform stack accesses into fixed register accesses. Becauseof this, that the JVM is a stack architecture does not imply a speed penalty for emulation on register-basedarchitectures when using a JIT compiler. In the face of the code-verified JVM architecture, it makes no difference toa JIT compiler whether it gets named imaginary registers or imaginary stack positions that must be allocated to thetarget architecture's registers. In fact, code verification makes the JVM different from a classic stack architecturewhose efficient emulation with a JIT compiler is more complicated and typically carried out by a slower interpreter.Code verification also ensures that arbitrary bit patterns cannot get used as an address. Memory protection isachieved without the need for a memory management unit (MMU). Thus, JVM is an efficient way of gettingmemory protection on simple architectures that lack an MMU. This is analogous to managed code in Microsoft's.NET Common Language Runtime, and conceptually similar to capability architectures such as the Plessey 250, andIBM System/38.

Bytecode instructionsThe JVM has instructions for the following groups of tasks:• Load and store• Arithmetic• Type conversion• Object creation and manipulation• Operand stack management (push / pop)• Control transfer (branching)• Method invocation and return• Throwing exceptions• Monitor-based concurrencyThe aim is binary compatibility. Each particular host operating system needs its own implementation of the JVM andruntime. These JVMs interpret the bytecode semantically the same way, but the actual implementation may bedifferent. More complex than just emulating bytecode is compatibly and efficiently implementing the Java core APIthat must be mapped to each host operating system.

Secure execution of remote codeA virtual machine architecture allows very fine-grained control over the actions that code within the machine ispermitted to take. This is designed to allow safe execution of untrusted code from remote sources, a model used byJava applets. Applets run within a VM incorporated into a user's browser, executing code downloaded from a remoteHTTP server. The remote code runs in a restricted "sandbox", which is designed to protect the user frommisbehaving or malicious code. Publishers can purchase a certificate with which to digitally sign applets as "safe",giving them permission to ask the user to break out of the sandbox and access the local file system, clipboard ornetwork.

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C to bytecode compilersFrom the point of view of a compiler, the Java Virtual Machine is just another processor with an instruction set, Javabytecode, for which code can be generated. The JVM was originally designed to execute programs written in theJava language. However, the JVM provides an execution environment in the form of a bytecode instruction set and aruntime system that is general enough that it can be used as the target for compilers of other languages.Because of its close association with the Java language, the JVM performs the strict runtime checks mandated by theJava specification. That requires C to bytecode compilers to provide their own "lax machine abstraction", forinstance producing compiled code that uses a Java array to represent main memory (so pointers can be compiled tointegers), and linking the C library to a centralized Java class that emulates system calls. Most or all of the compilerslisted below use a similar approach.Several C to bytecode compilers exist:• NestedVM [8] translates C to MIPS machine language first before converting to Java bytecode.• Cibyl [9] works similarly to NestedVM but targets J2ME devices.• LLJVM [10] compiles C to LLVM IR, which is then translated to JVM bytecode.• C2J [11] is also GCC-based, but it produces intermediary Java source code before generating bytecode.[12]

Supports the full ANSI C runtime.• Axiomatic Multi-Platform C [13] supports full ANSI C 1989, SWT, and J2ME CDC 1.1 for mobile devices.• Java Backend for GCC [14], possibly the oldest project of its kind, was developed at The University of

Queensland in 1999.• Javum [15] is an attempt to port the full GNU environment to the JVM, and includes one of the above compilers

packaged with additional utilities.• egcs-jvm [16] appears to be an inactive project.Compilers targeting Java bytecode have been written for other programming languages, including Ada and COBOL.

LicensingStarting with J2SE 5.0, changes to the JVM specification have been developed under the Java Community Process asJSR 924.[17] As of 2006, changes to specification to support changes proposed to the class file format (JSR 202[18] )are being done as a maintenance release of JSR 924. The specification for the JVM is published in book form,[19]

known as "blue book". The preface states:We intend that this specification should sufficiently document the Java Virtual Machine to make possiblecompatible clean-room implementations. Oracle provides tests that verify the proper operation ofimplementations of the Java Virtual Machine.

Oracle's JVM is called HotSpot. Clean-room Java implementations include Kaffe, IBM J9 and Dalvik. Oracle retainscontrol over the Java trademark, which it uses to certify implementation suites as fully compatible with Oracle'sspecification.

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HeapThe Java Virtual Machine heap is the area of memory used by the JVM (and specifically HotSpot) for dynamicmemory allocation.[20] The heap is split up into "generations":• The young generation stores short-lived objects that are created and immediately garbage collected.• Objects that persist longer are moved to the old generation (also called the tenured generation).• The permanent generation (or permgen) is used for class definitions and associated metadata.[21] [22]

Originally there was no permanent generation, and objects and classes were stored together in the same area. But asclass unloading occurs much more rarely than objects are collected, moving class structures to a specific area allowssignificant performance improvements.[21]

Notes[1] http:/ / www. java. com/ en/ about/ Learn about Java Technology[2] "The java and javaw commands" (http:/ / publib. boulder. ibm. com/ infocenter/ realtime/ v2r0/ index. jsp?topic=/ com. ibm. rt. doc. 20/ user/

java. html). IBM. 2010. . Retrieved 2011-01-16. "The javaw command is identical to java, except that javaw has no associated consolewindow."

[3] 1996, possibly the first new language specifically designed to run on the JVM)[4] Tolksdorf, Robert (2005). "Languages for the Java VM" (http:/ / www. is-research. de/ info/ vmlanguages/ ). . Retrieved 2008-06-08.[5] Nutter, Charles (2007-01-03). "InvokeDynamic: Actually Useful?" (http:/ / headius. blogspot. com/ 2007/ 01/ invokedynamic-actually-useful.

html). . Retrieved 2008-01-25.[6] Krill, Paul (2008-01-31). "Sun's Da Vinci Machine broadens JVM coverage" (http:/ / www. infoworld. com/ article/ 08/ 01/ 31/

davinci-machine_1. html). . Retrieved 2008-02-06.[7] "The Verification process" (http:/ / java. sun. com/ docs/ books/ jvms/ second_edition/ html/ ClassFile. doc. html#9766). The Java Virtual

Machine Specification. Sun Microsystems. 1999. . Retrieved 2009-05-31.[8] http:/ / nestedvm. ibex. org/[9] http:/ / cibyl. googlecode. com[10] http:/ / da. vidr. cc/ projects/ lljvm/[11] http:/ / www. novosoft-us. com/ solutions/ product_c2j. shtml[12] http:/ / tech. novosoft-us. com/ product_c2j_faq. jsp[13] http:/ / objectmix. com/ compilers/ 37664-ampc-version-1-6-2-released-c-java-class-files-compiler. html[14] http:/ / www. itee. uq. edu. au/ ~cristina/ uqbt. html#gcc-jvm[15] http:/ / sourceforge. net/ projects/ javum/[16] http:/ / sourceforge. net/ projects/ egcs-jvm/[17] JSR 924 (http:/ / www. jcp. org/ en/ jsr/ detail?id=924), specifies changes to the JVM specification starting with J2SE 5.0[18] JSR 202 (http:/ / www. jcp. org/ en/ jsr/ detail?id=202), specifies a number of changes to the class file format[19] The Java Virtual Machine Specification (http:/ / java. sun. com/ docs/ books/ vmspec/ ) (the first (http:/ / java. sun. com/ docs/ books/

vmspec/ html/ VMSpecTOC. doc. html) and second (http:/ / java. sun. com/ docs/ books/ vmspec/ 2nd-edition/ html/ VMSpecTOC. doc. html)editions are also available online)

[20] "Frequently Asked Questions about Garbage Collection in the Hotspot Java Virtual Machine" (http:/ / java. sun. com/ docs/ hotspot/ gc1. 4.2/ faq. html). Sun Microsystems. 6 February 2003. . Retrieved 7 February 2009.

[21] Masamitsu, Jon (28 November 2006). "Presenting the Permanent Generation" (http:/ / blogs. sun. com/ jonthecollector/ entry/presenting_the_permanent_generation). . Retrieved 7 February 2009.

[22] Nutter, Charles (11 September 2008). "A First Taste of InvokeDynamic" (http:/ / blog. headius. com/ 2008/ 09/ first-taste-of-invokedynamic.html). . Retrieved 7 February 2009.

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References• Clarifications and Amendments to the Java Virtual Machine Specification, Second Edition (http:/ / java. sun. com/

docs/ books/ vmspec/ 2nd-edition/ jvms-clarify. html) includes list of changes to be made to support J2SE 5.0 andJSR 45

• JSR 45 (http:/ / www. jcp. org/ en/ jsr/ detail?id=45), specifies changes to the class file format to supportsource-level debugging of languages such as JavaServer Pages (JSP) and SQLJ that are translated to Java

External links• The Java Virtual Machine Specification (http:/ / java. sun. com/ docs/ books/ vmspec/ )• Java implementations (http:/ / www. dmoz. org/ Computers/ Programming/ Languages/ Java/ Implementations/ )

at the Open Directory Project• Sun to build virtual machine for iPhone - ComputerWorld (http:/ / www. computerworld. com/ action/ article.

do?command=viewArticleBasic& articleId=9067358)• Java Virtual Machine Download Link (http:/ / java. com/ en/ download/ inc/ windows_new_xpi. jsp)• JVM implementation in pure Java (http:/ / igormaznitsa. com/ projects/ mjvm/ index. html)

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Article Sources and Contributors 7

Article Sources and ContributorsJava Virtual Machine  Source: http://en.wikipedia.org/w/index.php?oldid=412758984  Contributors: @modi, A3csc300, AWendt, Aavviof, Abarnea 2000, Abdull, Abu badali, Ahoerstemeier,Alainr345, Aldie, Alifakoor, Alphachimp, Alphax, AnOddName, Andre Engels, Angitha.t, Anwar saadat, Arrenlex, Atramos, Axlq, Bawolff, BeauPaisley, Blue Em, Boing! said Zebedee, BryanDerksen, Burschik, CALR, Cactus.man, Calm, Chrismear, ClashTheBunny, Classicalecon, CodeMonk, Congruence, Conversion script, CyberSkull, Cyp, Dalvizu, Damian Yerrick, Danhicks,DataWraith, Dave Cohoe, David Gerard, DavidCary, Dbabbitt, Debackerl, Dejvid, Delirium, Derek farn, Dnstest, Doradus, Doug Bell, DougsTech, Dreish, Ds13, Epbr123, Fram, Frap,Fromageestciel, Furrykef, Gazpacho, Ghettoblaster, Gtoal, Gurklurk, Gwernol, Haakon, Hairy Dude, Happysailor, Harborsparrow, Harp, Hdante, Headius, Hervegirod, Hfastedge, Hirzel, Hu12,Iane, Igoldste, Intgr, J E Bailey, JWB, Jabman, JacobBramley, JamesMLane, Jason.grossman, Jerryobject, Jim88Argentina, Jj137, Joaopaulo1511, Joehni, John of Reading, Johnuniq,JokerXtreme, Jondel, Karl Dickman, Khalid hassani, Kishonti, Kks krishna, Krishan.sunbeam, Kuasha, Kungfuadam, Lee J Haywood, Lerdthenerd, Liberty Miller, Livy the pixie,LoStrangolatore, Lumingz, Ma8thew, Maniaphobic, Mariolina, Martijn Hoekstra, MattGiuca, Mav, Mboverload, Mfc, Miko3k, Mjscud, Moa3333, Modster, Moppet65535, Napi, Neilc, Nemti,Ng.j, Nickg, NoIdeaNick, Nuno Tavares, Oleg Alexandrov, Ortolan88, Paul Foxworthy, Pegasus1138, Phyzome, Plasticup, Poccil, Quaysea, Rakesh.usenet, Randolf Richardson, Raven4x4x,RedWolf, Rich Farmbrough, Rursus, SAE1962, SF007, SJP, Sander123, Schoeberl, Serketan, Shadowjams, Sixth Estate, Slamb, SlowByte, Smarkflea, Smjg, Smyth, Snydeq, Square87, StephenB Streater, Stw, Swing, Tagus, Takis, TakuyaMurata, The Anome, Thumperward, TimTay, TittoAssini, ToddFincannon, Toussaint, Tyomitch, UU, Underpants, Unsungzeros, VasilievVV,Versus22, Vicarious, Vinny, VladimirReshetnikov, Vy0123, Weregerbil, Whiner01, Whispering, Wikicrusader, Winterst, Woohookitty, Wootery, Wrelwser43, Xevious, Yar, Yozh, 275anonymous edits

LicenseCreative Commons Attribution-Share Alike 3.0 Unportedhttp:/ / creativecommons. org/ licenses/ by-sa/ 3. 0/